Remove gaps from $wgFileExtensions array

If $wgFileExtensions contains values that are in $wgFileBlacklist,
when Setup.php uses array_diff to remove them it leaves $wgFileExtensions
as an array with nonconsecutive integer indices.

When this array is encoded by the Xml class, for use in client-side
JavaScript, the nonconsecutive indices cause it to encode it as an
Object rather than Array.  This breaks the extension processing in
UploadWizard's JS code.  To fix this, use array_values to remove the
gaps in indices.

Bug: 44776
Change-Id: I4ef7f1de90a0384800722839f3fa3a581c63bac9
Merge "Remove gaps from $wgFileExtensions array"